Recently, a liquid crystal display device is used in many fields. For example, a digital meter using such a liquid crystal display device is provided on an instrument panel of automobiles. The liquid crystal display device is generally provided most of its illumination by an illuminating apparatus. Such an illuminating apparatus is referred to as a back-illuminating apparatus. The back-illuminating apparatus must illuminate uniformly the overall surface of the liquid crystal display device.
The back-illuminating apparatus is typically constructed using a low pressure mercury vapor discharge lamp such as a fluorescent lamp and a casing for housing the low pressure mercury vapor discharge lamp. The casing has a light diffusion plate on one side and a reflector on the other side. The illuminating apparatus is applied to the liquid crystal display device so that the light diffusion plate faces toward the liquid crystal display device. Thus, the light diffusive transmission plate diffusively transmits both the light directly radiated from the low pressure mercury vapor discharge lamp and the light reflected from the reflector. The diffused light from the light diffusion plate is uniformly applied to the overall surface of the liquid crystal display device.
The low pressure mercury vapor discharge lamp has a higher illuminating efficiency, less heat generation and a longer life in comparison to incandescent bulbs. Moreover, the low pressure mercury vapor discharge lamp has a relatively large light-emitting area due to its long discharge path. Also, the shape of the discharge path of the low pressure mercury vapor discharge lamp may be freely formed, for instance, into a U-letter shape, a W-letter shape, etc. Thus, the illuminating apparatus using the low pressure mercury vapor discharge lamp is advantageously able to perform a uniform illumination for the liquid crystal display device.
However, the low pressure mercury vapor discharge lamp takes a relatively long time to reach a rated brightness. It especially takes a long time in a low ambient temperature. This is because the mercury sealed in the low pressure mercury vapor discharge lamp does not vaporize as well in low ambient temperatures. Furthermore, the light diffusion plate and the reflector severely absorb the heat of the discharge lamp when they are close to each other for reducing the size of the illuminating apparatus.
For example, automobiles are required to operate in an ambient temperature range from approximately +40.degree. C. to approximately -30.degree. C. Therefore, the low pressure mercury vapor discharge lamp is required to start rapidly and to operate in such a low ambient temperature, e.g., -30.degree. C.
Conventionally, a heater is provided on the low pressure mercury vapor discharge lamp for quickening the start of the operation. Such a heater is attached to the low pressure mercury vapor discharge lamp at a side where the heater does not face to the light diffusion plate. Generally, such a heater is placed on the side where the heater faces the reflector. As a result, the heater accelerates the evaporation of the mercury in the low pressure mercury vapor discharge lamp without disturbing the illumination for the liquid crystal display device.
In a conventional illuminating apparatus, a laminate heater is attached on the low pressure mercury vapor discharge lamp by a tubular fixing member such as a thermo-shrinkage tube. The thermo-shrinkage tube keeps the low pressure mercury vapor discharge lamp warm. However, when the low pressure mercury vapor discharge lamp is formed into a complicated shape, such as a U-letter shape, a W-letter shape, etc., it is difficult to make the shape of the laminate heater conform to the shape of the low pressure mercury vapor discharge lamp. It is also difficult to fit the thermo-shrinkage tube on the low pressure mercury vapor discharge lamp for fixing the laminate heater.
In another conventional illuminating apparatus, a print circuit type heater is coated on the low pressure mercury vapor discharge lamp. This type heater is formed by a resistive material such as a silver (Ag) paste. An illuminating apparatus of this type is disclosed in Japanese Patent Disclosure Tokkai-Sho 56-120066.
In the latter conventional illuminating apparatus, the printed heater can be easily conformed to the shape of the low pressure mercury vapor discharge lamp, even if the low pressure mercury vapor discharge lamp is formed in a complicated shape.
However, the surface of the low pressure mercury vapor discharge lamp not coated with the printed heater is exposed, so that low pressure mercury vapor discharge lamps of this type are difficult to keep warm. When gaps between the low pressure mercury vapor discharge lamp and the light diffusion plate and the reflector are decreased to allow thinning of the size of the illuminating apparatus, the sides of the low pressure mercury vapor discharge lamp facing the light diffusion plate and the reflector are undesirably cooled. Thus, this latter type of conventional illuminating apparatus is also not able to operate quickly at low ambient temperatures.
In addition, the prior art illumination devices suffer from a flaw in that the light they emit is often not properly diffused. This often results in bright and dark portions, or shadows, when even illumination is required.